Developmental response to brain inflammation

Perinatal inflammation contributes to neurodevelopmental diseases, and animal models have revealed that the inflammatory response within the central nervous system is age dependent. It remains unclear what intrinsic and/or extrinsic factors are responsible for this variation. Here, my aim was to discover the mechanisms responsible for the age-dependent changes in the inflammatory response of the brain by injecting interleukin-1 (IL-1β) into the brain of mice at postnatal day (P)7, P14, P21 or into adult mice. A "window of susceptibility" was found at P14, which was associated with marked neutrophil recruitment and blood-brain barrier (BBB) breakdown, in response to a low dose of IL-1β. Evaluation of cytokine, chemokine, and adhesion molecule mRNA transcripts failed to reveal any specific increases in basal or reactive expression following the injection of IL-1β at P14. The extrinsic hepatic acute phase response (APR) was evaluated, but, once again, there was no evidence that an altered APR might account for the enhanced inflammatory response at P14. Indeed, an inverse relationship between the magnitude of the leukocyte recruitment to the brain and the APR was discovered. Enhancement of the APR with intravenous IL-1β after injection of a low dose of IL-1β into the brain was found to reduce the number of neutrophils and BBB permeability in the brain. While no molecular changes seem to account for the presence of the "window of susceptibility", a population of Iba-1+ large, flattened and irregular perivascular cells was discovered within the P14 brain, that may contribute to the increased leukocyte recruitment at P14. Although variations in the brain inflammatory response with development were not fully account for, my results highlight the importance of the systemic inflammatory response on the outcome of acute brain injury and suggest that the systemic APR might be manipulated therapeutically to protect the brain in the perinatal period.